This invention relates to spectro-photometers adapted to be used as a multi-purpose spectro-photometer or a detector for a liquid chromatograph. More particularly, this invention relates to multi-channel type spectro-photometers having an array of photo-diodes as a detector.
The basic structure of a prior art multi-channel type spectro-photometer provided with an array of photo-diodes will be explained first with reference to FIG. 9, as comprising a light source 1, a sample cell 4, a converging lens 2 for collecting light from the light source 1 and guiding it to the sample cell 4, a shutter 3, a slit 5 at the entrance to a spectroscope, a concave grating 6 serving as the light-dispersing element of the spectroscope, a photo-diode array 7 disposed at the exit of the spectroscope to serve as a multi-channel detector, a signal processing and control circuit 10 (herein referred to as the control unit) and a memory 11. The concave grating 6 is adapted not only to disperse the light transmitted through the sample cell 4 but also to focus the image of the slit 5 on the light-receiving surface of the photo-diode array 7. The photo-diode array 7 may comprise, for example, 500 aligned light-receiving elements such that the light with wavelength 200 nm transmitted through the sample cell 4 will be received by the first light-receiving element on the light-receiving surface of the photo-diode array 7, that the light with wavelength 700 nm will be received by the 500th light-receiving element and that light with wavelength within the range of 200 nm to 700 nm can be detected by the elements therebetween. The shutter 3 is for carrying out dark current adjustments of the photo-diode array 7. When a dark current adjustment is carried out, the shutter 3 is closed to provide a dark condition, and the photo-diode array 7 is scanned by means of a driving circuit (not shown) to measure the dark current value of each light-receiving element, and the measured dark current values are stored in the memory 11. Next, the shutter 3 is opened to introduce light in order to determine the background condition of the absorption spectrum. The photo-diode array 7 is scanned to measure the signal from each light-receiving element, and the background spectrum is obtained by subtracting the dark current value of each light-receiving element from the measured value. The background spectrum thus obtained is also stored in the memory 11.
Next, a sample is injected into the sample cell 4, the shutter 3 is opened, and the light from the light source 1 is focused by the converging lens 2 and led to the sample cell 4. The light transmitted through the sample cell 4 is passed through the slit 5 and dispersed by the concave grating 6 such that the image of the slit 5 is on the surface of the photo-diode array 7. The photo-diode array 7 is scanned under this condition, and the signal from each light-receiving element is inputted to the control unit 10 for effecting dark current and background corrections to obtain the absorption spectrum of the sample.
The photo-diode array 7 may be structured as shown in FIG. 10 wherein numeral 21 indicates a shift register, numerals 22 indicate photo-diodes in the array 7, numerals 24 indicate capacitors each connected in parallel with an associated one of the photo-diodes 22, and numerals 23 indicate switches each connected in series with an associated one of the parallel connections of a photo-diode 22 and a capacitor 24. If light is made incident on each of the photo-diodes 22 while each of the capacitors 24 is in a charged condition, the capacitors 24 are discharged by the photoelectric effect. If the shift register 21 switches on the switches 23 sequentially one at a time at the frequency of a clock pulse as shown schematically in FIG. 10 and the electric charge on each capacitor 24 is measured, one can obtain the electric charge discharged from each capacitor 24. By repeating this measurement, one can obtain the amount of light which was made incident on each photo-diode 22 from the measured electric charge.
Prior art spectro-photometers of the multi-channel type were structured as explained above, and the series of measurements by the photo-diode array was controlled by the start pulse and the clock pulse as shown in FIG. 10, and the periods of these two pulses were set such that none of the elements would exceed the saturation charge value, or the limit value of discharge. In the case of a liquid chromatograph, for example, use may be made of both a deuterium lamp and a tungsten lamp as light source in order to cover a wide range of wavelength. Since the emission spectra of these two lamps are as shown in FIG. 11 by dotted and chain lines, respectively, and since these two lamps are turned on together, the total emission spectrum is as shown by the solid line in FIG. 11. In other words, the intensity varies significantly within the useful range of wavelength. As explained above, however, every light-receiving element of the photo-diode is exposed to light for a same period of time, while the saturation charge of photoelectric converter elements of a charge-accumulating type is finite. In order to prevent saturation, the time for charge accumulation must be limited. In the example of FIG. 11, the wavelength at which the intensity is the strongest is indicated by arrow A, and the charge-accumulating time is limited according to this maximum intensity. On the other hand, major sources of noise for photo-diodes are circuit noise and reset noise, but their magnitudes are constant, independent of the signal intensity. Thus, if the charge-accumulating time period is set such that there will be no saturation at the wavelength corresponding to the largest intensity within the range of wavelength of the light source, signals with wavelengths corresponding to smaller light intensities will be too small compared to the noise.
The sensitivity of a spectro-photometer as described above deteriorates with time because the quantity of light emitted from its light source decreases, as shown in FIG. 12 for a deuterium lamp. For this reason, a guarantee period is normally specified for each lamp, as indicated in FIG. 12, such that the user will be sure to exchange the lamp after its guarantee period has been exceeded. For a deuterium lamp, the guarantee period is about 2000 hours. If such a deuterium lamp is used for 8 hours every day, for example, it must be replaced within less than a year.
The sensitivity of such a spectro-photometer deteriorates also due to a change in the optical system such as when the slit is made narrower and the quantity of incident light is thereby reduced. In the case of a photometer using a tungsten lamp and a deuterium lamp as described above with reference to FIG. 11, its sensitivity will drop, for example, if only the tungsten lamp is used for measurements because the total quantity of light drops (from the solid line to the chain line as shown in FIG. 11).